1. Field of the Invention
This invention lies in the field of liquid and gaseous fuel burning. More particularly, this invention concerns fuel burning apparatus in which a minimum value of NOx is provided in the effluent gases.
Still more particularly, this invention is concerned with fuel burning with low NOx and with control of the general shape of the flame as concerns its length and width.
2. Description of the Prior Art
Burning of all fuels is productive of oxides of nitrogen (Nox) in normal operation. Such oxides of nitrogen as are produced in combination with olefinic hydrocarbons, which may be present in the atmosphere, provide a source of smog.
Smog is recognized universally as potentially damaging to animal tissue. Consequently, severe limitations on the Nox content of stack gases vented to the atmosphere as the result of fuels burning, have been imposed by various government authorities and agents.
The prior art is best represented by U.S. Pat. No. 4,004,875. This patent has been the basis of a wide application of low NOx burners. However, when firing rate changes significantly, such as from 100% to 80%, as is typical of daily process heater firing, there is difficulty in maintaining NOx suppression. The reason for this is that, at reduced firing rate, the furnace draft remains constant, or approximately so, and increased air to fuel ratios destroy the less-than-stoichiometric burning zone prior to tertiary air delivery, which results in less-than-optimum NOx reduction, plus higher-than-desirable excess air.
What is required is a burner which provides means for correction of any condition of firing, such as might be required when the furnace draft remains substantially constant while changes in firing rate are made. If such corrections can be made, the result is the continuation of NOx suppression and the maintenance of optimum excess air for high thermal efficiency. In the prior art burner there is no control of the tertiary air, which is caused to flow by furnace draft, while the primary and secondary air also flow for the same reason. The total air flow will vary as the square root of the furnace draft. Thus, only one rate of fuel burning or firing rate, at a condition of furnace draft, will provide required excess air and NOx suppression. This would seem to indicate that control of air flow would provide some benefit.
What is not immediately evident is, that the air entry control must be proportionately controlled for maintenance of a less-than-stoichiometric burning zone prior to entry of tertiary air to the less-than-stoichiometric gases, for completion of fuel burning plus preferred excess air when the firing rate is caused to vary. If the conditions as outlined are maintained, there is suitable NOx suppression in any condition of draft and firing rate and furnace excess air remains best for high thermal efficiency. This is to say that control must be proportional and simultaneous for primary, secondary and tertiary air for best and most assured operation in all firing conditions.